1. Field of the Invention
The present invention relates to the material of an insulating film necessary for fabricating a thin film transistor (TFT), and a method of manufacturing the insulating film material. More particularly, the present invention is well suited for application to an electro-optical device which is typified by a liquid crystal display panel or an electro-luminescence (EL) display device of active matrix type wherein pixel units and driver circuits are disposed on an identical substrate, and to an electronic equipment in which such an electro-optical device is installed. Incidentally, here in this specification, an expression xe2x80x9csemiconductor devicexe2x80x9d is intended to signify general devices which can function by utilizing semiconductor properties, and it shall cover within its category, an electro-optical device which is typified by a liquid crystal display device of active matrix type fabricated using thin film transistors, and an electronic equipment in which such an electro-optical device is installed as a component.
2. Prior Art
There has been developed a thin-film transistor (hereafter referred to as a TFT), having an active layer made from a crystalline semiconductor film, which is crystallized by a method such as laser annealing or thermal annealing from an amorphous semiconductor film, formed on an insulating substrate having light transparency characteristics, such as a glass. The substrate mainly used in the manufacture of the TFT is a glass substrate such as a barium-borosilicate glass or alumino-borosilicate glass. This type of glass substrate has inferior resistance to heat when compared with a quartz substrate, but has the advantages of a low market price, and the fact that a large surface area substrate can be easily manufactured.
The structure of the TFT can be roughly divided into a top gate type and a bottom gate type, with respect to the arrangement of a gate electrode. In the top gate type, an active layer is formed on an insulating substrate such as a glass substrate, and a gate insulating film and a gate electrode are formed in order on the active layer. Furthermore, there are many cases in which a base film is formed between the substrate and the active layer. On the other hand, a gate electrode is formed on a similar substrate in the bottom gate type, and a gate insulating film and an active layer are formed in order on the gate electrode. In addition, a protective insulating film or an interlayer insulating film is formed on the active layer.
The gate insulating film of the TFT is manufactured from a film such as a silicon oxide film, a silicon nitride film, or a silicon oxynitride film. The reason that these types of materials are used is because in order to form a good interface with respect to an amorphous silicon film or a crystalline silicon film forming the active layer, it is preferable to form the insulating films from a material having silicon as one of the principal constituents.
It is considered as preferable to manufacture the above insulating films by plasma CVD or low pressure CVD. Plasma CVD is a technique of decomposing a raw material gas in a glow discharge, forming a radical (meaning here a chemically activated one) by being made into a plasma, and depositing this on the substrate. In plasma CVD, it is possible to deposit a film at a low temperature of normally 400xc2x0 C. or less. However, ions also exist within the plasma, and therefore it is necessary to skillfully control the damage to the substrate due to ions accelerated by an electric field occurring in a sheath region. On the other hand, low pressure CVD is a method of thermally decomposing a raw material gas and depositing a film on the substrate. There is no damage to the substrate due to ions, as with plasma CVD, but low pressure CVD has the disadvantage of slow deposition speed, so it cannot always be applied to manufacturing process in view of circumstances.
It is required of a gate insulating film to sufficiently lower an interface state density and a defect level density (bulk defect density) in the film. It is also required to consider an internal stress and the magnitude of change thereof attributed to a heat treatment. It is important for forming the gate insulating film of good quality to prevent the introduction of interfaces and defects into the film in the course of the deposition of the film, and to prepare a composition adapted to lower the defect level density of the formed film. Expedients each of which employs a starting gas exhibiting a high efficiency of decomposition, have been thought out for that purpose. By way of example, a silicon oxide film which is manufactured by plasma CVD with a mixed gas consisting of TEOS (Tetraethyl Ortho Silicate whose chemical formula is Si(OC2H5)4) and oxygen (O2) is the insulating film of good quality. It has been known that, when a MOS structure is fabricated using the silicon oxide film and is subjected to a BT (Bias Temperature) test, the fluctuation of a flat-band voltage (hereinbelow, expressed by xe2x80x9cVfbxe2x80x9d) can be diminished to:a practicable degree.
Since, however, water (H2O) is liable to be produced and is easily entered into the film in the course of decomposing the TEOS by glow discharge, thermal annealing needs to be performed at 400xc2x0 C.-600xc2x0 C. after the formation of the film in order to obtain the good quality film as stated above. Unfavorably it becomes a factor for the increase of a fabrication cost to incorporate such a high-temperature annealing step into the fabricating process of a TFT.
On the other hand, a silicon nitride film which is manufactured from, for example, SiH4, NH3 and N2 by plasma CVD can offer a dense and hard film. Since, however, the silicon nitride film has a high defect level density and a high internal stress, it gives rise to a distortion at its interface defined with an active layer. Accordingly, it exerts the bad influences of shifting a threshold voltage (hereinbelow, expressed by xe2x80x9cVthxe2x80x9d) and enlarging a sub-threshold constant (hereinbelow, shortly termed xe2x80x9cS valuexe2x80x9d), on the characteristics of a TFT.
Further, a silicon oxynitride film which is manufactured by plasma CVD with a mixed gas consisting of SiH4 and N2O can offer a film of high density in such a way that severalxe2x80x94several tens atomic % of nitrogen is contained in the film. Under some manufactural conditions, however, defect levels due to Sixe2x80x94N bonds appear, and the value of a voltage Vfb fluctuates greatly in a BT test. Even when the film is stable in the BT test, it lacks in a thermal stability, and the voltage Vfb is caused to fluctuate by a heat treatment at 300xc2x0 C.-550xc2x0 C. Such fluctuations in the characteristics can be conjectured ascribable to the change of the composition of the silicon oxynitride film.
Meanwhile, there has been known a technique wherein a silicon oxynitride film is manufactured by plasma CVD with a mixed gas consisting of SiH4, N2O and H2. By way of example, a thesis xe2x80x9cStructural and optical properties of amorphous silicon oxynitridexe2x80x9d, Jiun-Lin Yeh and Si-Chen Lee, Journal of Applied Physics, vol. 79, No. 2, pp.656-663, 1996, discloses hydrogenated silicon oxynitride films which were manufactured by plasma CVD in which a decomposing temperature was set at 250xc2x0 C., the mixing ratio of hydrogen (H2) to SiH4+N2O was held constant at 0.9 to 1.0, and the mixing ratio of SiH4 and N2O as expressed by Xg=[N2O]/([SiH4]+[N2O]) was changed from 0.05 to 0.975. inclusive. With Fourier-transform infrared spectrometry (FT-IR), however, it was clearly observed that HSixe2x80x94O3 bonds and H2Sixe2x80x94O2 bonds existed in the hydrogenated silicon oxynitride films manufactured here. Such bonds are conjectured, not only to exhibit inferior thermal stabilities, but also to form defect levels around the bonds due to the fluctuations of coordination numbers. In such a case, despite the silicon oxynitride film, unless the composition thereof or the constituents thereof including impurity elements is/are examined in detail, the film cannot be easily used for a gate insulating film which exerts serious influences on the characteristics of a TFT.
The present invention consists in techniques for solving the problems as stated above, and it has for its object to provide a gate insulating film which is suitable for insulated gate type transistors typified by a TFT, and a method of manufacturing the gate insulating film. Another object of the invention is to ensure the stability and reliability of the characteristics of a TFT, such as the threshold voltage (Vth) and sub-threshold constant (S value) thereof, by employing such a gate insulating film.
In order to solve the problems, according to the invention, a silicon oxynitride film is manufactured using SiH4, N2O and H2 by plasma CVD, and this film is applied to the gate insulating film of a TFT. The characteristics of the silicon oxynitride film to be manufactured are controlled chiefly by changing the flow rates of N2O and H2. A hydrogen concentration and a nitrogen concentration in the film can be increased within the above range by the increase of the flow rate of H2. Besides, the hydrogen concentration and the nitrogen concentration in the film can be decreased to heighten an oxygen concentration by the: increase of the flow rate of N2O. On the other hand, a silicon concentration is hardly changed even when only the ratio between the gas flow rates of H2 and N2O is changed.
Concretely, a silicon oxynitride film is formed within the ranges of Xh=0.5-5 (Xh=H2/(SiH4+N2O)) and Xg=0.94-0.97 (Xg=N2O/(SiH4+N2O)) in terms of the ratios among the flow rates of SiH4, N2O and H2, while a silicon oxynitride film is formed within the ranges of Xh=0 (Xh=H2/(SiH4+N2O)) and Xg=0.97-0.99 (Xg=0.97-0.99 (Xg=N2O/(SiH4+N2O)). These silicon oxynitride films are properly used.
When the silicon oxynitride film is to be manufactured by the plasma CVD, H2 is added to a mixed gas consisting of SiH4 and N2O, whereby radicals produced from SiH4 by decomposition can be prevented from polymerizing in a vapor phase (in a reaction space), to nullify the production of particles. Moreover, in the growing surface of the film, it is possible to prevent excessive hydrogen from being introduced into the film, owing to the reaction of pulling out surface adsorption hydrogen as based on hydrogen radicals. Such an action correlates closely with the temperature of a substrate during the deposition of the film, and it can be attained by holding the substrate temperature at 300xc2x0 C.-450xc2x0 C., preferably 400xc2x0 C. As a result, a dense film of low defect density can be formed, and a slight amount of hydrogen contained in the film acts effectively to relieve a lattice distortion. In order to heighten the generation density of hydrogen radicals by decomposing hydrogen molecules, the frequency of a high-frequency power source for generating glow discharge is set within a range of 13.56 MHz-120 MHz, preferably 27 MHz-60 MHz, and the discharge power density thereof is set at 0.1-1 W/cm2.
Owing to the adoption of the manufactural conditions mentioned above, the silicon oxynitride film according to the present invention is endowed with a composition in which a nitrogen concentration is at least 0.1 atomic % and less than 15 atomic %, a hydrogen concentration is at least 0.1 atomic % and less than 5 atomic %, and an oxygen concentration is at least 50 atomic % and less than 70 atomic %.
The feature of the invention consists, in a case where the gate insulating film of a TFT is formed of a silicon oxynitride film, the composition of the silicon oxynitride film is made different on, at least, the active layer side and gate electrode side of the gate insulating film so as to become high in the nitrogen concentration and hydrogen concentration of the film and low in the oxygen concentration thereof relatively on the active layer side.
By way of example, that first layer of the gate insulating film which lies in touch with an active layer is formed of a silicon oxynitride film which has a nitrogen concentration of 2-15 atomic %, a hydrogen concentration of 1.5-5 atomic % and an oxygen concentration of 50-60 atomic %, and that second layer of the gate insulating film which lies in touch with a gate electrode is formed of a silicon oxynitride film which has a nitrogen concentration of 0.1-2 atomic %, a hydrogen concentration of 0.1-2 atomic % and an oxygen concentration of 60-65 atomic %, thereby to establish a stepped concentration gradient. Alternatively, the composition may well be continuously changed without the definite distinction between the first and second layers as stated above.
The gate insulating film of such a construction is applicable to either a TFT of top gate type or a TFT of bottom gate type (or inverse stagger type).
The silicon oxynitride film according to the present invention is manufactured by plasma CVD with a starting gas which consists of SiH4, N2O and H2. Here will be explained capacitance-voltage characteristics (hereinbelow, abbreviated to xe2x80x9cC-V characteristicsxe2x80x9d) which are attained when samples of MOS structures are manufactured using the silicon oxynitride film.
A plasma equipment which has a construction of capacitance-coupled parallel plate scheme is employed for the manufacture of the silicon oxynitride film. Otherwise, it is allowed to employ a plasma CVD equipment which is of inductive coupling type or which conjointly uses the energy of a magnetic field as in an electron cyclotron resonance. The silicon oxynitride film can have its composition changed by employing SiH4 and N2O gases and further adding H2 thereto. During the plasma manufacture, a pressure is set at 10 Pa-133 Pa (preferably, 20 Pa-40 Pa), a high-frequency power density at 0.2 W/cm2-1 W/cm2 (preferably, 0.3 W/cm2-0.5 W/cm2), a substrate temperature at 200xc2x0 C.-450xc2x0 C. (preferably, 300xc2x0 C.-400xc2x0 C.), a the oscillation frequency of, a high-frequency power source at 10 MHz-120 MHz (preferably, 27 MHz-60 MHz).
Three kinds of manufactural conditions are listed in Table 1. Conditions #210 are the manufactural conditions of a silicon oxynitride film which is formed from SiH4 and N2O. On the other hand, conditions #211 and #212 are the manufactural conditions in the case where H2 is added to the SiH4 and N2O gases, and where the flow rate of the additional H2 is changed. Here in this specification, the silicon oxynitride film manufactured from SiH4 and N2O shall be expressed as xe2x80x9csilicon oxynitride film (A)xe2x80x9d, and the silicon oxynitride film manufactured from SiH4 and N2O with H2 added thereto shall be expressed as xe2x80x9csilicon oxynitride film (B)xe2x80x9d. More specifically, the silicon oxynitride film (A) is formed within the ranges of Xh=0 (Xh=H2/(SiH4+N2O)) and Xg=0.97-0.99 (Xg=N2O/(SiH4+N2O)) in terms of the ratios among the flow rates of SiH4, N2O and H2, while the silicon oxynitride film (B) is formed within the ranges of Xh=0.5-5 (Xh=H2/(SiH4+N2O)) and Xg=0.94-0.97 (Xg=N2O/(SiH4+N2O)) in terms of the ratios among the flow rates of SiH4, N2O and H2.
Also, the conditions of preprocessing which is performed before the formation of the silicon oxynitride film are listed in Table 1. Although the preprocessing is not indispensable, it is useful for enhancing the reproducibility of the characteristics of the silicon oxynitride film itself and that of these characteristics in the case of the application to the TFT.
Referring to Table 1, the preprocessing is performed for 2 minutes by generating a plasma under the conditions of a hydrogen introducing flow rate of 200 SCCM, a pressure of 20 Pa and a high-frequency power density of 0.2 W/cm2. Alternatively, the preprocessing may well be performed in such a way that a plasma is similarly generated by introducing hydrogen at 100 SCCM and oxygen at 100 SCCM. Further, although not indicated in the table, the preprocessing may well be performed for several minutes under the conditions of a pressure of 10 Pa-70 Pa and a high-frequency power density of 0.1 W/cm2-0.5 W/cm2 by introducing N2O and hydrogen. During such preprocessing, the temperature of a substrate may be held at 300xc2x0 C.-450xc2x0 C., preferably 400xc2x0 C. The preprocessing has the function of cleaning the surface of the substrate for deposition, and the function of adsorbing hydrogen on the substrate surface for deposition so as to temporarily inactivate this surface, thereby to stabilize the interface properties of a hydrogenated silicon oxynitride film which is to be deposited later. Besides, when oxygen and N2O are simultaneously introduced, such a favorable function is fulfilled that the outermost part of the substrate surface for deposition and the vicinity thereof are oxidized to lower an interface state density.
Concretely, the sample of the hydrogenated silicon oxynitride film (B) was manufactured under the film forming conditions #211; an SiH4 flow rate of 5 SCCM, an N2O flow rate of 120 SCCM and a hydrogen flow rate of 500 SCCM, a reaction pressure of 20 Pa, a high-frequency power density of 0.4 W/cm2, and a substrate temperature of 400xc2x0 C. A high-frequency power source frequency may be 10 MHz-120 MHz, preferably 27 MHz-60 MHz, and it was set at 60 MHz here. Besides, the other sample was manufactured under the conditions #212 in which the flow rate of hydrogen was changed to 125 SCCM from the conditions #211. Regarding the flow rates of the individual gases, their absolute values are not restricted, but their ratios are significant. Letting Xh denote [H2]/([SiH4]+[N2O]), this ratio Xh may be set within a range of 0.1-7. Also, letting Xg denote [N2O]/([SiH4]+[N2O]) as stated before, this ratio Xg may be set within a range of 0.90-0.996. Besides, the film forming conditions of the silicon oxynitride film (A) are the conditions #210.
The typical characteristics of the silicon oxynitride films manufactured under such conditions are listed in Table 2. This table indicates the compositions, hydrogen (H), nitrogen (N), oxygen (O) and silicon (Si) of the films as measured by Rutherford Backscattering Spectrometry (which shall be abbreviated to xe2x80x9cRBSxe2x80x9d below, and which used a system xe2x80x9c3S-R10xe2x80x9d, an accelerator xe2x80x9cNEC3SDH pelletronxe2x80x9d and an end station xe2x80x9cCEandA RBS-400xe2x80x9d), the densities of the films, and the initial values of the internal stresses of the films and the values of the internal stresses after thermal annealing (obtained with a measuring instrument xe2x80x9cModel-30114xe2x80x9d manufactured by Ionic System Inc.). In mentioning the internal stresses, the sign (+) denotes a tensile stress (a stress by which the film is deformed inside), and the sign (xe2x88x92) denotes a compressive stress (a stress by which the film is deformed outside).
The results of Table 2 reveal that the concentration of hydrogen contained in the film is increased by adding H2 during the film formation. Consequently, the contents of oxygen and nitrogen are changed. In the silicon oxynitride film (A), the ratio of O to Si is 1.9 (1.7-2 as an allowable range), and the ratio of N to Si is 0.04 (0.02-0.06 as an allowable range). In contrast, in the silicon oxynitride film (B) whose composition changes depending upon the flow rate of H2 added during the film formation, the ratio of O to Si is about 1.6 (1.4-1.8 as an allowable range), and the ratio of N to Si is 0.14-0.18 (0.05-0.5 as an allowable range), whereby the proportion of O decreases relative to Si, and that of N increases.
The increase of the nitrogen content corresponds to increase in the density of the film, and the nitrogen content of 6.5 atoms/cm3 in the silicon oxynitride film (A) increases to the nitrogen content of 7.1 atoms/cm3 in the silicon oxynitride film (B), so that the film (B) is densified. Such a change in the density is demonstrated in terms of the etching rates of a mixed solution (trade name xe2x80x9cLAL500xe2x80x9d produced by Stella-Chemifa Co.), which ,contains 7.13% of ammonium hydrofluoride (NH4HF2) and 15.4% of ammonium fluoride (NH4F), at 20xc2x0 C. That is, as indicated in Table 1, the etching rate is 120 nm/min in the silicon oxynitride film (A), whereas it is 63 nm/min-105 nm/min in the silicon oxynitride film (B). Thus, the film (B) is densified.
Further, in terms of the internal stresses, regarding the silicon oxynitride film (A), a compressive stress of xe2x88x924.26xc3x97108 Pa is greatly changed to xe2x88x927.29xc3x97106 Pa by a heat treatment (at 500xc2x0 C. for 1 hour +at 550xc2x0 C. for 4 hours: equivalent to processing conditions at the step of crystallization). On the other hand, regarding the silicon oxynitride film (B), a tensile stress of +2.31xc3x97108 Pa is exhibited, and it is hardly changed even by the heat treatment. The phenomenon that the internal stress is changed by the heat treatment, can be considered in association with the structural change and compositional change of the film, and it signifies that the thermal stability to the stress of the silicon oxynitride film (A) is inferior.
The samples of an MOS structure were fabricated using the silicon oxynitride films manufactured on the basis of the conditions of Table 1, and the C-V (capacitance versus voltage) characteristics of the samples and the fluctuations of the flat-band voltages Vfb of the samples as attributed to BT (Bias Temperature) tests were investigated. It is the most desirable that the voltage Vfb becomes 0 V in the C-V characteristics, and that it is not fluctuated even by the BT test. The shift of the Vfb value from 0 V signifies that a defect level density is high at the interface of the film or within the film. Each of the samples was fabricated as stated below. The silicon oxynitride film was formed on a single-crystal silicon substrate (of CZ-N type having a crystal face of  less than 100 greater than  and a resistivity of 3-7 [xcexa9 cm]) to a thickness of 100 nm-150 nm under the conditions indicated in Table 1. Aluminum (Al) for an electrode was deposited to a thickness of 400 nm by sputtering, and an electrode area was set at 78.5 mm2 mm. Besides, an Al electrode was formed on the back surface of the single-crystal silicon substrate at the same thickness. The resulting substrate structure was sintered by performing a heat treatment at 350xc2x0 C. for 30 minutes in a hydrogen atmosphere. Each of the BT tests was such that the resulting structure was let stand at 150xc2x0 C. for 1 hour in a state where a voltage of +1.7 MV (or xe2x88x921.7 MV) was applied to the electrode overlying the silicon oxynitride film. Here in this specification, for the sake of convenience, the case of applying a minus voltage shall be expressed as xe2x80x9cxe2x88x92BT testxe2x80x9d, and the case of applying a plus voltage as xe2x80x9c+BT testxe2x80x9d.
First, the C-V characteristics of the respective silicon oxynitride films (A) and (B) were evaluated. Each sample had the silicon oxynitride film (A) or (B) of 130 nm which was formed on the single-crystal silicon substrate under the manufactural conditions of Table 1. The measurement of the C-V characteristics was such that the initial value after the fabrication of the sample was found, and that the subsequent values were found after the xe2x88x92BT test and the +BT test, and after the further heat treatment (at 500xc2x0 C. for 1 hour+at 550xc2x0 C. for 4 hours). Table 3 lists the results of the measurement in terms of the values of the flat-band voltages Vfb. Incidentally, the manufactural conditions of the samples mentioned in Table 3 correspond to those in Table 1. A model xe2x80x9cYHP-4192Axe2x80x9d fabricated by Yokogawa Hewlett-Packard Company was employed for the measurement of the C-V characteristics.
The sample #210 is the silicon oxynitride film (A). Whereas the initial value of the voltage Vfb is xe2x88x921.6 V, the subsequent values are fluctuated down to xe2x88x923.3 V by the BT tests. However, the Vfb values are hardly changed by the heat treatment of the specified conditions. Whereas the Vfb values of the samples #211 and #212 are hardly changed by the BT tests, they are fluctuated in the plus direction by the heat treatment. Besides, when the initial values of the voltage Vfb are compared, the initial value of the sample #212 being the silicon oxynitride film (B) is the closest to 0 V, and hence, this sample is suitable.
It can be judged from the results of Table 3 that, on the basis of the initial Vfb value, the sample #212 of the silicon oxynitride film (B) is suitable for forming the interface with a semiconductor. The change of the voltage Vfb of this sample due to the heat treatment will be caused by the emission of hydrogen from the film, etc. On the other hand, considering a thermal stability, the silicon oxynitride film (A) can be judged suitable.
Next, there were fabricated and evaluated samples each of which had a two-layer structure consisting of the silicon oxynitride film (A) and the silicon oxynitride film (B) and in which the order of stacking the films from the side of a semiconductor surface was changed. Concretely, the samples each of which had the structure of a single-crystal silicon substrate   the silicon oxynitride film (A)   the silicon oxynitride film (B) were classified as xe2x80x9csamples-Axe2x80x9d, while the samples each of which had the structure of the single-crystal silicon substrate   the silicon oxynitride film (B)   the silicon oxynitride film (A) were classified as xe2x80x9csamples-Bxe2x80x9d. The samples of each class having different film thicknesses were fabricated. Incidentally, the conditions #212 were adopted for the silicon oxynitride film (B). Table 4 lists the evaluated sample structures and the evaluated results. FIG. 27 is a graph showing the Vfb values of the samples. By the way, numerals affixed to the samples-A are for distinguishing the differences of the thicknesses of the stacked films. The same holds true of the samples-B.
The results in Table 4 and FIG. 27 reveal that, whereas the samples-A exhibit initial Vfb values of xe2x88x920.4 V-xe2x88x920.9 V, the samples-B exhibit initial Vfb values of 0 V-0.3 V which are favorable. Besides, after a BT test (after the application of a voltage of xe2x88x921.7 MV to an electrode overlying the silicon oxynitride film), whereas the samples-A exhibit Vfb values of xe2x88x920.8 V-xe2x88x921.6 V, the samples-B exhibit Vfb values of xe2x88x920.1 V-xe2x88x920.3 V which indicate a smaller fluctuating width and a higher stability.
In this manner, the clear differences are noted in the C-V characteristics of the samples having the structures listed in Table 4, and they indicate the existence of a structure which can make small both the initial value of the voltage Vfb and the fluctuated value thereof after the BT test. That is, they indicate that the structure in which the silicon oxynitride film (B) is first deposited on the single-crystal silicon substrate and is overlaid with the silicon oxynitride film (A) is good.
In the above, the typical examples have been mentioned on the characteristics of the silicon oxynitride films. Of course, the silicon oxynitride films which are insulating films employable in the present invention are not restricted to those mentioned in Tables 1-4 and FIG. 27. The composition of the silicon oxynitride film (A), which is a gate insulating film suitable for a semiconductor device typified by a TFT, is set at a nitrogen concentration of 0.1-2 atomic %, a hydrogen concentration of 0.1-2 atomic % and an oxygen concentration of 60-65 atomic %. On the other hand, the composition of the silicon oxynitride film (B) is set at a nitrogen concentration of 2-15 atomic %, a hydrogen concentration of 1.5-5 atomic % and an oxygen concentration of 50-60 atomic %. Further, the density of the silicon oxynitride film (A) is set to be at least 6xc3x971022 and less than 7xc3x971022 atoms/cm3, while the density of the silicon oxynitride film (B) is set to be at least 7xc3x971022 and less than 8xc3x971022 atoms/cm3. The aforementioned etching rate with the mixed solution which contains ammonium hydrofluoride (NH4HF2) and ammonium fluoride (NH4F), is set at 110-130 nm/min for the silicon oxynitride film (A) and at 60-110 nm/min for the silicon oxynitride film (B).